Acne vulgaris
While not life-threatening, acne vulgaris can cause significant problems for affected individuals. Depending on its severity and other factors, recalcitrant acne can be psychologically debilitating, and can impose significant financial and emotional costs on those whom it afflicts. Despite some recent successes in acne therapy, treatment failures are still common, especially in adult women. While many adults “outgrow” this disease, there are some who continue to be afflicted during much of adulthood, despite continued medical advances. Unfortunately, the most potent acne medication in current use is administered systemically via a treatment that is teratogenic, an important issue for many women. There is an unfilled need for a more localized and effective treatment for acne, one with minimal side effects.
Acne, which most commonly occurs during adolescence, is influenced by several factors. The pathology centers on the pilosebaceous follicle, comprising the sebaceous gland, the follicle (pore), and the vellus hair. Factors that promote the formation of comedones (whiteheads or blackheads) include the following: (1) increased sebum production, (2) inflammation of the dermis and follicles by inflammatory mediators, (3) hyperkeratinization and obstruction of the upper region of the follicle, and (4) colonization of the follicle by the bacterium Propionibacterium acnes,
Adolescence is marked by an increase in levels of circulating androgens, particularly dehydroepiandrosterone sulfate (DHEAS). The increased androgen levels are thought to cause sebaceous glands to enlarge and to increase sebum production. While most acne patients have normal hormone levels, there are reasons to conclude that increased sebum production plays an important role in acne. For example, there is a correlation between the rate of sebum production and the severity of acne. In addition, acne patients typically produce sebum that is deficient in linoleic acid, which is a potential cause of abnormal keratinization and follicular obstruction.
In response to increased sebum levels, Propionibacterium acnes, a gram positive, anaerobic, diphtheroid bacterium, often colonizes the sebaceous follicles. P. acnes exacerbates acne by acting as a chemo-attractant for neutrophils (a type of white blood cells, also called polymorphonuclear leukocytes, or PMNs). The neutrophils ingest the P. acnes, and in doing so release various hydrolytic enzymes that damage the follicular wall. The released follicular contents then invade the dermis and cause an inflammatory reaction, manifesting as pustules, erythematous papules, or nodules. In a separate route, P. acnes can hydrolyze triglycerides to free fatty acids, which also increase inflammation and follicular obstruction. P. acnes may also activate the complement components of the immune system, which can also lead to follicular obstruction.
The follicles are lined with squamous epithelium, a layer of cells that is contiguous with the skin surface. In an acne-prone individual, the shedding of cells from this lining is often impeded, perhaps due to an increased level of intracellular adhesion that promotes the retention of cells. The retained cells can obstruct the follicles, resulting in comedones. The exact cause of this inhibited shedding is unknown, but it may be related either to abnormalities in epidermal differentiation, or to abnormal sebum composition (e.g., a deficiency in linoleic acid).
It has also been demonstrated that increased sebum levels can irritate keratinocytes, causing the release of interleukin-1, which in turn can cause follicular hyperkeratinization.
The final common pathway in each of these acne-causing routes, which are not mutually exclusive, is follicular obstruction.
Current Acne Therapies
Currently-used acne therapies are directed at various aspects of the acne cascade. The most commonly used therapy is probably topical benzoyl peroxide, which has an antibacterial effect, and which may also decrease free fatty acids, resulting in a decrease in inflammation and follicular obstruction.
Topical and systemic antibiotics have been used to target P. acnes. The antibiotics that have been used for this purpose include erythromycin, tetracycline, clindamycin, and doxycycline. Predictably, the prolonged use of antibiotics often leads to the development of resistant strains of P. acnes.
Topical and systemic retinoids (derivatives of vitamin A) have been used to normalize the keratinization of the follicle, decreasing follicular obstruction and rupture. Systemic isotretinoin (Accutane™), which is highly effective for acne, unfortunately has serious adverse effects, most notably teratogenicity. Topical retinoids include tretinoin (Retin-A™), whose chemical structure is similar to that of isotretinoin.
The current state of the art in treating acne is summarized, for example, in the following two review articles: D. Krowchuk, “Managing Acne in Adolescents,” Pediatric Dermatology, vol. 47, pp. 841-857 (2000); and B. Johnson et al., “Use of Systemic Agents in the Treatment of Acne Vulgaris,” American Family Physician, vol. 62, pp. 1823-1830, 1835-1836 (2000).
Advances in acne therapy have followed better understanding of its multiple causes. Most treatments are directed at normalizing keratin production (e.g., through the use of retinoids), or at controlling bacterial colonization. To the inventors' knowledge, no prior treatments have sought to affect either sweat gland production or keratinocyte locomotion, both of which can be factors in the follicular occlusion that leads to acne.
Botulinum Toxins and Pharmacology
Clostridium botulinum, an anaerobic bacterium, produces seven toxins that have similar neurotoxic effects, but that are antigenically distinct: serotypes A through G. These toxins are potent neuroparalytic agents that inhibit the release of acetylcholine at neuromuscular junctions and at neuroglandular junctions. See M. F. Brin, “Botulinum toxin: Chemistry, Pharmacology, Toxicity, and Immunology,” Muscle and Nerve Supp. 6, pp. S146-S168 (1997); and 1. Kinkelin et al., “Effective Treatment of Frontal Hyperhidrosis with botulinum toxin A,” Brit. J. Dermatol., vol. 143, pp. 824-827 (2000).
Neither the synthesis nor the storage of acetylcholine is affected by botulinum toxin. Therefore, its effects are temporary. Botulinum toxin penetrates the endosomal membrane into the cytosol, where the secretion of acetylcholine is blocked. Botulinum toxin also appears to possess anti-inflammatory effects. Studies of the effect of botulinum toxin on glandular function have observed that its effects occur at the presynaptic terminal of the parasympathetic cholinergic nerves.
Botulinum toxin A (sold under the trademark Botox® by Allergan (Irvine, Calif.), and under the trademark Dysport® by Ipsen Limited (Maidenhead, Berkshire, United Kingdom)) has been used for certain therapeutic purposes at both neuroglandular and neuromuscular junctions. Its temporary neuromuscular blockade has found numerous previous uses, including the treatment of strabismus, migraines, achalasia, hemifacial spasm, rhytids, and spastic dysphonia. The injections have proven to be safe and effective for these purposes, with only minor side effects such as local swelling and transient weakness of nearby muscles. Botulinum toxins A and B have been reported to be well tolerated in patients who have used it.
Intracutaneous botulinum toxin A has become a primary therapy for axillary, palmar, solar, and frontal hyperhidrosis (excessive sweating), as well as for gustatory sweating (Frey's Syndrome). It has also been shown to be an effective alternative to surgery in Hailey-Hailey disease, a benign familial pemphigus that affects primarily the groin and axillary folds (areas of high moisture).
Studies have found no long-term effects of botulinum toxin A in human tissue. Histological studies have found no nerve fiber degeneration, and no sweat gland atrophy as a result of therapy. Antibodies to botulinum toxin A have been found in fewer than 5% of patients receiving botulinum toxin A injections. There have been no reported cases of anaphylaxis in response to injections of botulinum toxin A.
Botulinum toxin B (sold under the trademark Myobloc™ and NeuroBloc™ by Elan Pharmaceuticals (Dublin, Ireland)) has been approved by FDA for use in treating patients with cervical dystonia.
All seven botulinum toxin serotypes (A through G) are available commercially from Metabiologics, Inc. (Madison, Wis.).
Neuromuscular Junction Blockade
The first reported clinical use of botulinum toxin A was its use in the treatment of strabismus in the 1970's. Injecting the small extraocular muscles resulted in a realignment of the muscles, straightening of the globe, and improvement of visual alignment in some cases. Subsequently, botulinum toxin A has been used therapeutically for several other purposes, both functional and cosmetic. It has proven effective in treating focal dystonias, spasmodic dysphonia, blepharospasm, hemifacial spasm, torticollis, and cervical dystonia. It has also recently been used in facial plastic surgery, especially to treat rhytids (wrinkles).
In 1990, the use of botulinum toxin A was reported in ambulatory and nonambulatory cerebral palsy patients, namely by injecting different muscle groups to treat spastic diplegia. For example, double-blinded, placebo-controlled studies on the effect of botulinum toxin A on limb dystonias and spasticity have found significant subjective as well as objective benefits.
The American Academy of Ophthalmology, the American Academy of Neurology, the American Academy of Otolaryngology, and the National Institutes of Health have all released statements supporting the therapeutic efficacy of botulinum toxin A for a variety of clinical conditions. Among the conditions for which botulinum toxin A has been used are blepharospasm, strabismus, cervical dystonia, spasmodic torticollis, rhytids, hemifacial spasm, facial spasm, spasmodic dysphonia, focal hand dystonia, hyperfunctional facial wrinkles, Frey's syndrome, hyperhidrosis, adult spasticity, adjunctive treatment of spasticity in cerebral palsy, oromandibular dystonia, and other dystonias.
Neuroglandular Junction Blockade
The effects of botulinum toxin A as an anticholinergic agent at the neuroglandular junction have not been explored as extensively as those occurring at the neuromuscular junction. Clinical studies examining the effect of intracutaneous botulinum toxin for focal hyperhidrosis found complete abolition of sweating in the injected area within 3 to 7 days. No adverse effects were reported, and in a five month follow-up there were no clinical recurrences of the hyperhidrosis. See generally I. Kinkelin et al., “Effective Treatment of Frontal Hyperhidrosis with botulinum toxin A,” Brit. J. Dermatol., vol. 143, pp. 824-827 (2000).
Gustatory sweating is another area of neuroglandular dysfunction in which botulinum toxin A has proven effective. Gustatory sweating (or Frey's syndrome) is a disabling disorder in which the cheek skin sweats profusely during eating. The syndrome often occurs after parotidectomy, and may be due to the misdirection of the regenerating parasympathetic fibers that enervate the sweat glands of the face. Intracutaneous botulinum toxin A has been reported to significantly decrease or prevent sweating for over six months, with no clinical evidence of facial weakness in any patients.
Botulinum toxin A injected into the submandibular glands has been reported to significantly decrease salivation resulting from stimulation of the lingual nerves. The decreased salivation was temporary, and did not appear to be directly toxic to the acinar cells of the gland. See D. Suskind et al., “Clinical study of botulinum A toxin in the treatment of sialorrhea in children with cerebral palsy,” Laryngoscope, vol. 112, pp. 73-81 (2002).
Canine studies have also shown that vasomotor rhinorhea, a parasympathetically controlled phenomenon, responds to topical botulinum toxin A.
While the duration of botulinum toxin A's action at the neuromuscular junction appears to be about three months, there appears to be a longer-lasting effect at the glandular level. Botulinum toxin A has produced anhydrosis for over 12 months in patients with gustatory sweating. The reason for the difference in duration of action is uncertain. Hypotheses include a higher rate of re-synthesis of SNAP-25 (the protein cleaved by botulinum toxin) in neuromuscular synapses, and a higher area of axonal sprouting and consecutive reinervation of muscle fibers as compared to that in glandular tissue.
To the inventors' knowledge, there have been no prior suggestions that botulinum toxin might be used in the treatment or prevention of acne.